A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To image smaller features, it has been proposed to use extreme ultraviolet radiation (EUV) with a wavelength in the range of 5-20 nanometers, in particular, 13.5 nanometers, or a charged particle beam, e.g. an ion beam and an electron beam, as the exposure radiation in a lithographic apparatus. These types of radiation need the beam path in the apparatus to be evacuated to avoid absorption. Since there are no known materials suitable to make a refractive optical element for EUV radiation, EUV lithographic apparatus use mirrors in the radiation, illumination and projection systems. Such mirrors are highly susceptible to contamination, thereby reducing their reflectivity and hence the throughput of the apparatus. Further, sources for EUV may produce debris whose entry into the illumination system should be minimized.
In order to reduce the chance of debris entering the illumination system, contaminant traps may be used. Such traps are disposed in the radiation system downstream of the source. The traps comprise elements that provide a surface on which debris can deposit. Conventional radiation systems may also comprise a collector which collects the radiation beam. It has been found that debris may also deposit on elements in the collector. The deposit of debris on the collector significantly reduces its operational lifetime before it must be cleaned.
It has been found that as the temperature of elements in the contaminant trap increases, the greater the contamination, and hence, the shorter the lifetime of the collector. This is because it has been found that at higher temperatures, the elements of the contaminant trap may become secondary sources of contamination. In particular, certain debris on the elements may be vaporized. The vaporized debris then goes on to further contaminate the collector. Further, in applying more powerful EUV sources causing increasing heat loads, contaminant trap elements may melt and/or vaporize, which may cause a collapse of the entire debris barrier.